docs: clarify Node version requirements

[asim48-ctrl]

why

The root package already declares the supported Node range as ^20.19.0 || >=22.12.0, but the source-build docs only said Node.js. That leaves contributors guessing which Node versions match the package engines, especially around the Node support discussion in #1995.

what changed

• added the existing package engine range to the root source-build instructions
• added the same Node range to the server-v4 local API prerequisites

test plan

• git diff --check

No runtime tests; docs-only change.

---

Summary by cubic

Clarified Node.js version support in the docs to match our engines. Added ^20.19.0 || >=22.12.0 to the root build-from-source instructions and the server-v4 local API prerequisites.

^{Written for commit 811f3c7. Summary will update on new commits. Review in cubic}

[changeset-bot]

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Latest commit: 811f3c7

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[github-actions]

This PR is from an external contributor and must be approved by a stagehand team member with write access before CI can run.
Approving the latest commit mirrors it into an internal PR owned by the approver.
If new commits are pushed later, the internal PR stays open but is marked stale until someone approves the latest external commit and refreshes it.

[cubic-dev-ai]

No issues found across 2 files

Confidence score: 5/5

• Automated review surfaced no issues in the provided summaries.
• No files require special attention.

Architecture diagram

sequenceDiagram
    participant User
    participant Repo as Repository
    participant Pkg as Package Config
    participant Doc as Documentation
    
    Note over User,Doc: Node Version Requirement Flow
    
    User->>Repo: Clone & build from source
    Repo->>Pkg: Read engines field
    Pkg-->>Repo: Node ^20.19.0 || >=22.12.0
    alt Documentation check
        Repo->>Doc: root README.md
        Doc-->>Repo: Now includes Node version
        Repo->>Doc: packages/server-v4/README.md
        Doc-->>Repo: Now includes Node version
    end
    User->>Doc: Review documented Node requirements
    Note over User: Satisfies engine constraint
    
    alt Node version mismatch
        User->>Pkg: Validate local Node version
        Pkg-->>User: Version check fails (EENGINE)
        User->>Doc: See documented requirements
        Doc-->>User: Must install correct Node version
    end

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